Humans are primates, an order of mammals that also includes lemurs, lorises, monkeys and apes. In fact, humans are apes. We share more than 98 per cent of our DNA with chimpanzees and their close relatives, the bonobos.
Primates have highly dexterous hands, and many also have dexterous feet. In most, the thumb can be opposed to the other fingers, enabling them to grasp and manipulate objects – a prerequisite of tool use. Their eyes are large and face forward, giving them good binocular vision – essential for judging distances. Primates’ brains are relatively large compared with those of other animals, and this endows them with a great capacity to learn and adapt. The young stay with their mothers for longer than the young of many other animals, and have ample time to pick up skills and customs. Many primates live in complex social groups.
The first primate-like creatures emerged around the time of the extinction of the dinosaurs, 66 million years ago. They resembled squirrels or tree shrews in both size and appearance. The first true primates appeared some 10 million years later. Similar to lemurs and lorises, they spread to many parts of the world, but when the monkeys emerged 34 million years ago they were largely outcompeted. Today lemurs are confined to the island of Madagascar, which the monkeys never reached. The first apes appeared 23 million years ago, but the split between our ancestors and the ancestors of chimps and bonobos did not happen till about 7 million years ago.
By then, large areas of tropical forest had been replaced by more open woodland and savannah. With the change of environment there came a need to adapt to a ground-based rather than tree-based way of life. Around 6 million years ago, early humans began to walk some of the time on their hind legs. This ability, called bipedalism, meant they could look over the long grass to see both predators and prey. It also reduced the surface area of skin exposed to sunlight, and lengthened the stride, so that humans could cover greater distances. The effect was both to widen the range in which they could hunt and gather food, and to enable populations to migrate to different territories altogether.
Our closest relatives
We share 98.7 per cent of our DNA with chimpanzees and bonobos, but these two species differ in their behaviour. Chimps are male-dominated, hunt in groups, are aggressively territorial, and may kill other chimps. Only high-ranking males get to mate. Chimps use a variety of tools, for example to crack nuts or to catch ants. Tool use has only been observed among bonobos in captivity.
Bonobo groups are dominated by females (who have strong bonds with each other), although there is much less sexual differentiation than in chimps. The territories of different bonobo groups overlap, and they have not been observed hunting in groups. Sex is frequent between males and females, and with members of the same sex. Sex is important for social bonding and conflict resolution, not just for reproduction. This has been described as ‘sex for peace’.
Claiming thabt behaviour is genetically determined is always going to prove controversial. But it is certainly possible to see some aspects of human behaviour reflected in that of the chimps, while other aspects are closer to that of the bonobos.